The present invention relates to a power supply circuit and preferably one for providing both positive and negative DC voltage levels. The power supply of this invention may be used for many different applications but is primarily adapted for use with a microcomputer system.
One object of the present invention is to provide an improved power supply circuit having voltage regulation. In the preferred embodiment of the invention, at least one voltage regulator circuit is used having associated therewith pass transistor means, either in the form of a single transistor or a transistor pair.
Another object of the present invention is to provide an improved power supply circuit having current limiting means.
Still another object of the present invention is to provide an improved power supply circuit that is relatively simple in construction, that uses relatively few circuit components, and that yet provides a plurality of DC voltage levels in the usual logic level range.
To accomplish the foregoing and other objects of this invention, there is provided a power supply circuit that comprises means for receiving an unregulated input voltage, which voltage is typically at least on the order of 5 volts DC. Preferably, there is provided an on-off switch, which in the disclosed embodiment, has contacts for individually interrupting separate voltage lines coupling to each individual voltage circuit. Also, in the disclosed embodiment at the input there is provided a bridge circuit for providing full wave rectification. Furthermore, the power supply circuit of this invention may be operated from an AC adapter which can provide an unregulated DC voltage of, for example, 18 or 19 volts and an AC voltage which couples to the bridge circuit. The output of the bridge circuit preferably provides both positive and negative voltages for generating the voltage levels on the +5 volt bus and -5 volt bus. Of course, in accordance with the invention, many different voltage levels can be employed, including even a power supply for providing only a single voltage level. The circuit of the present invention has at least one voltage bus and a pass transistor means which may comprise either a single transistor or a pair of transistors. This pass transistor means has main electrodes intercoupling between the input to the circuit and the voltage bus. Furthermore, the pass transistor means has a control electrode which is controlled from the voltage regulator. In the disclosed embodiment, there is a regulator associated with two of the voltage busses. This voltage regulator comprises means for establishing a voltage reference, means for monitoring the voltage bus and the voltage reference, which is preferably an operational amplifier, and output control transistor means, having an output coupling to the control electrode of the pass transistor means. This regulator means may comprise a current source in series with a zener diode for providing the reference voltage while the output control transistor means may comprise a pair of transistors one of which is controlled externally of the voltage regulator. In this regard the power supply circuit of this invention also includes a current limiting transistor means having electrodes coupled to the reference voltage and the voltage bus and also having an output electrode for operating the control transistor means of the regulator to decrease current flow through the pass transistor means when the current limiting transistor means detects an increased current flow in the voltage bus. Preferably, there is provided in series with the pass transistor, a sensing resistor essentially connected in parallel with one main electrode of the pass transistor and the voltage bus. It is the voltage developed across this resistor that is sensed by the voltage regulator for maintaining a proper voltage level on the output bus.